For example, a wheel of an automobile is rotatably supported to a suspension device by a double row angular contact rolling bearing unit, or the like. Also, in order to ensure the running stability of the vehicle, for example, not only an anti-lock brake system (ABS) or a traction control system (TCS) but also a vehicle running stabilizing unit such as an electronic vehicle stability control system (ESC), or the like is employed. In order to control such vehicle running stabilizing unit, signals indicating a rotation speed of a wheel, accelerations applied to an automobile body in respective directions, and the like are required. Also, in order to apply the higher control, it is preferable in some cases that a magnitude of a load (one or both of a radial load and an axial load, for example) applied to the rolling bearing unit via the wheel should be known.
In view of such circumstances, in Patent Literature 1, the invention that measures a magnitude of a load applied to the rolling bearing unit by using a special encoder is set forth. FIGS. 14 to 16 show a first example of the conventional structure concerning a state measuring apparatus for a rolling bearing unit, in which the same load measuring principle as the structure set forth in Patent Literature 1 is employed. In this first example of the conventional structure, a hub 2 is supported rotatably on the inner diameter side of an outer ring 1 via a plurality of rolling elements 3, 3 such that the hub 2 is rotated together with a wheel while this hub attaches the wheel in use and also the outer ring 1 is not rotated in use. A contact angle of back-to-back arrangement type and also a preload are applied to these rolling elements 3, 3. In this case, balls are employed as the rolling elements 3, 3 in the illustrated example, but sometimes tapered rollers are employed in place of the balls in the case of a bearing unit for automobile whose weight is increased.
Also, a cylindrical encoder 4 is attached to an inner end portion of the hub 2 (the term “inner” in an axial direction denotes a center side in a width direction of an automobile in a fitted status to the automobile, i.e., rightward side in FIGS. 1, 2, 7 to 12, 14, 17, 18, 20, 24. In contrast, the term “outer” in the axial direction denotes the outward side in the width direction of the automobile in a fitted status to the automobile, i.e., leftward side in FIGS. 1, 2, 7 to 12, 14, 17, 18, 20, 24. Ditto with the recitation throughout this specification) concentrically with the hub 2. Also, a sensor holder 7 containing a pair of sensors 6a, 6b therein is held on an inner side of a cylindrical cover 5 with a bottom, which blocks an inner end opening of the outer ring 1. Also, sensing portions of these sensors 6a, 6b are opposed closely to outer peripheral surfaces of the encoder 4 acting as sensed surfaces.
This encoder 4 is made of a magnetic metal plate. Through holes 8, 8 and column portions 9, 9 (see FIG. 15) are arranged in a circumferential direction alternately at an equal interval in the front half portion (the inner half portion in the axial direction) of the outer peripheral surface of the encoder 4, which act as the sensed surface. The boundaries between the through holes 8, 8 and the column portions 9, 9 are inclined by the same angle to the axial direction (the width direction) of the sensed surface respectively, and also the inclination angles to the axial direction are set to the opposite direction mutually at the boundary located in an intermediate portion of the sensed surface in the axial direction. Accordingly, the through holes 8, 8 and the column portions 9, 9 constitute a “V”-shape, the intermediate portion of which in the axial direction protrudes mostly in the circumferential direction, respectively. Then, out of outer half portions in the axial direction and inner half portions in the axial direction of the sensed surface, in which the inclination directions at the boundaries are different mutually, the outer half portion in the axial direction is denoted as a first characteristic changing portion 10, and the inner half portion in the axial direction is denoted as a second characteristic changing portion 11. In this case, respective through holes constituting both characteristic changing portion 10, 11 may be formed either in a state that they are formed continuously mutually, as illustrated, or in a state that they are formed individually mutually (respective through holes may be arranged like almost V shapes whose intersection points are located discretely).
Also, the cover 5 is made of a metal plate such as a stainless steel plate, or the like and shaped as a whole into a cylinder with a bottom, and is fitted in and fixed to the inner end portion of the outer ring 1. This cover 5 has a cylinder portion 12 whose outer end portion is securely fitted to the inner end portion of the outer ring 1 (in the illustrated example, fitted into the inner end portion as the tight fitting), and a bottom plate portion 13 blocking an inner end opening of the cylinder portion 12. Also, the sensor holder 7 is made of a synthetic resin and is shaped as a whole into a cylinder with a bottom. The sensor holder 7 is equipped with a cylinder portion 14 for holding both sensors 6a, 6b, and a bottom plate portion 15 for blocking an inner end opening of the cylinder portion 14 and holding a sensor circuit (not shown). Such sensor holder 7 is formed by molding a synthetic resin in the cover 5 by means of injection molding (mold forming) or by fitting the sensor holder 7, which is formed previously by the injection molding, into the cover 5 and then fixing the sensor holder 7 to the cover 5 by adhesive 16. When the sensor holder 7 is fixed by the adhesive 16, the cylinder portion 14 is fitted into the cylinder portion 12 of the cover 5 without play, and a side surface of the bottom plate portion 15 (right side surface in FIG. 14) of the sensor holder 7 is fixed to a side surface of the bottom plate portion 13 (left side surface in FIG. 14) of the cover 5 by the adhesive 16.
Also, a pair of sensors 6a, 6b are constructed by a permanent magnet, and a magnetic sensing element such as Hall IC, Hall element, MR element, GMR element, or the like constituting a sensing portion respectively. A sensing portion of one sensor 6a is opposed closely to the first characteristic changing portion 10 and a sensing portion of the other sensor 6b is opposed closely to the second characteristic changing portion 11 in a state that a pair of sensors 6a, 6b are embedded integrally in a part of the cylinder portion 14 constituting the sensor holder 7 in the circumferential direction (for example, the lower end portion). The positions where the sensing portions of both sensors 6a, 6b are opposed to both characteristic changing portions 10, 11 respectively are set to the same positions of the encoder 4 in the circumferential direction. Also, the setting positions of respective members are restricted such that mostly projected portions of the through holes 8, 8 and the column portions 9, 9 in the circumferential direction at the intermediate portion in the axial direction (portions at which the inclination direction of the boundary is changed) are located just in the center positions between the sensing portions of both sensors 6a, 6b in a neutral status that no axial load is applied between the outer ring 1 and the hub 2.
In the case of the state measuring apparatus for the rolling bearing unit constructed as above, when the axial load is applied between the outer ring 1 and the hub 2 (the outer ring 1 and the hub 2 are relatively displaced in the axial direction), phases in which output signals of both sensors 6a, 6b are changed are shifted. That is, the sensing portions of both sensors 6a, 6b are opposed to portions on solid lines a, a in (A) of FIG. 16, i.e., portions that are deviated from the mostly projected portion to the same extent in the axial direction respectively, in a neutral status that no axial load is applied between the outer ring 1 and the hub 2. Therefore, phases of the output signals of both sensors 6a, 6b coincide with each other, as shown in (C) of FIG. 16.
In contrast, when the axial load is applied downward in (A) of FIG. 16 to the hub 2 to which the encoder 4 is fixed, the sensing portions of both sensors 6a, 6b are opposed to portions on broken lines b, b in (A) of FIG. 16, i.e., portions that are deviated from the mostly projected portion to the different extent in the axial direction respectively. In this status, phases of the output signals of both sensors 6a, 6b are shifted differently, as shown in (B) of FIG. 16. Then, when the axial load is applied upward in (A) of FIG. 16 to the hub 2 to which the encoder 4 is fixed, the sensing portions of both sensors 6a, 6b are opposed to portions on chain lines c, c in (A) of FIG. 16, i.e., portions whose deviations from the mostly projected portion in the axial direction are mutually different in the opposite direction respectively. In this status, phases of the output signals of both sensors 6a, 6b are shifted in the opposite direction to that in above (B), as shown in (D) of FIG. 16.
As described above, in the case of the state measuring apparatus for the rolling bearing unit known in the prior art and as set forth in Patent Literature 1, etc., phases of the output signals of both sensors 6a, 6b are shifted in the direction that responds to the acting direction (the relatively displacing direction between the outer ring 1 and the hub 2 in the axial direction) of the axial load applied between the outer ring 1 and the hub 2. Also, the extent to which the phases of the output signals of both sensors 6a, 6b are shifted by the axial load (relative displacement) is increased as the axial load (relative displacement) is increased much more. As a result, a direction and a magnitude of the relative displacement in the axial direction and an acting direction and a magnitude of the axial load, which are a status variable between the outer ring 1 and the hub 2 respectively, can be derived based on whether or not the phases of the output signals of both sensors 6a, 6b are shifted and the direction and a magnitude of the phase shifts when the phase shifts are present. In this case, processes of calculating such status variables are executed by a calculator (not shown). For this purpose, relations between a phase difference and the relative displacement in the axial direction or the load, which are examined in advance by theoretical calculations and experiments, are stored in a memory of this calculator in a form of relational expressions and maps.
In the case of the above first example of the conventional structure, the through holes 8, 8 and the column portions 9, 9 are arranged alternately on the sensed surface of the encoder 4, and the characteristic of the sensed surface are changed alternately at an equal interval. In contrast, as shown in FIG. 17, the state measuring apparatus for the rolling bearing unit equipped with an encoder 4a made of a permanent magnet in which the S pole and the N pole are arranged alternately on the outer peripheral surface as the sensed surface is set forth in Patent Literature 2 and is known in the prior art. The basic structure and action of the state measuring apparatus for the rolling bearing unit shown in FIG. 17 are similar to those in the above first example of the conventional structure shown in FIGS. 14 to 16. In the case of the second example of the conventional structure shown in FIG. 17, a permanent magnet is provided on the encoder 4a side, and hence a magnetic sensing element may be provided in principle on the sensors 6a, 6b side, and the permanent magnet is not needed. Also, in the case of the structure shown in FIG. 17, a hub main body constituting a hub 2a and an inner ring are coupled by a caulking portion that is formed on an inner end portion of the hub main body in the axial direction, instead of the nuts shown in FIG. 14. However, such structure is well known in the prior art and does not constitute a gist of the present invention.
Also, in Patent Literature 3, a state measuring apparatus for a rolling bearing unit as shown in FIGS. 18 to 21 are set forth. First, in the third example of the conventional structure shown in FIGS. 18 and 19, slit-like through holes 8a, 8a and column portions 9a, 9a (see FIG. 19) are arranged alternately at an equal interval in the circumferential direction in the front half portion of a cylindrical encoder 4b, which is made of a magnetic metal plate and is securely fitted to the inner end portion of the hub 2. Boundaries between the through holes 8a, 8a and the column portions 9a, 9a are inclined linearly by the same angle to the axial direction of the encoder 4b in the same direction respectively. Also, the sensing portions of a pair of sensors 6a, 6b, which are attached to the inner end portion of the outer ring 1 via the cover 5 and the sensor holder 7, are opposed closely to two locations of an outer peripheral surface, which acts as the sensed surface, in a vertical direction (portions whose phases in the circumferential direction are different mutually by 180 degree) of the front half portion of the encoder 4b. 
In the case of a rolling bearing unit for supporting the wheel of the automobile, the axial load applied between the outer ring 1 and the hub 2 is input from a contacting surface between an outer peripheral surface of a tire, which constitutes the wheel coupled/fixed to this hub 2, and road surface. Since this contacting surface exists on an outer side from a center of rotation of the outer ring 1 and the hub 2 in the radial direction, the axial load is not applied as pure axial load between the outer ring 1 and the hub 2 but is applied together with a moment in a virtual plane (in the vertical direction) containing center axes of the outer ring 1 and the hub 2 and a center of the contacting surface. When this moment is applied between the outer ring 1 and the hub 2, the center axis of the hub 2 is inclined to the center axis of the hub 2. Accordingly, the upper end portion of the encoder 4b is displaced in any direction with respect to the axial direction, and also the lower end portion of the same encoder is displaced in the opposite direction. As a result, phases of the output signals of both sensors 6a, 6b whose sensing portions are opposed closely to upper and lower end portions of the outer peripheral surface of the encoder 4b respectively are shifted in the opposite direction from the neutral position respectively. Therefore, the direction and the magnitude of the axial load can be derived based on the direction and the magnitude of the phase shift of the output signals of both sensors 6a, 6b. 
In the case of a fourth example of the conventional structure shown in FIGS. 20 and 21, through holes 8b, 8b and column portions 9b, 9b (see FIG. 21) are arranged alternately at an equal interval in the circumferential direction in the front half portion of a cylindrical encoder 4c, which is made of a magnetic metal plate and is securely fitted to the inner end portion of the hub 2. The through holes 8b, 8b are shaped into a trapezoid respectively when viewed from the radial direction, and the width dimension along the circumferential direction is changed gradually in the axial direction respectively. Also, the sensing portion of one sensor 6 being attached via the cover 5 and the sensor holder 7 is opposed closely to the outer peripheral surface, which acts as the sensed surface, of the front half portion of the encoder 4c. In the case of the fourth example of the conventional structure constructed in this manner, when the outer ring 1 and the hub 2 are displaced relatively in the axial direction based on the axial load, a duty ratio (high potential duration time/one period) of an output signal of the sensor 6 is changed. Therefore, not only a magnitude of the relative displacement but also a magnitude of the axial load can be derived based on this duty ratio.
Here, in the case of the first and second examples of the conventional structure shown in above FIGS. 14 to 17, only a set of sensors consisting of a pair of sensors 6a, 6b whose sensing portions are opposed to the first and second characteristic changing portions 10, 11 respectively are provided. In contrast, although not illustrated, in Patent Literature 3 and Patent Application No. 2006-345849, such a structure is set forth that displacements or external forces in multiple directions are sensed by providing a plurality of sensor sets each of which is composed of a pair of sensors.
By the way, in the case of the conventional structure and the structure of the prior invention, a thermal expansion or a thermal contraction is caused in respective constituent members in use due to changes in a heating value in the bearing portion and an environmental temperature. As a result, in the first and second examples of the conventional structure shown in FIGS. 14 to 17, there is such a possibility that, for example, as shown in (A)→(B) of FIG. 22, the sensed surface of the encoder 4 and the sensing portions of a pair of sensors 6a, 6b should be relatively displaced in the axial direction (the vertical direction in FIG. 22) like the case where an axial load is changed. When such relative displacement is caused, a phase difference existing between the output signals of both sensors 6a, 6b is changed, as indicated by a solid line→a broken line in FIG. 23. In this manner, when the phase difference is changed according to the thermal expansion or the thermal contraction (for another reason from that the axial load is changed), a zero point of the relationship that exists between the phase difference and the status variation (a value of the phase difference in a state that the radial load is not applied) is deviated. For this reason, an error is caused in a measured result of the above status variable according to this deviation. The error produced by such cause can be reduced by correcting a zero point on the side of the calculator while measuring a surrounding temperature of the bearing portion, or the like. In this case, from a viewpoint of keeping a measuring accuracy, it is not preferable that a change of the phase difference caused due to the thermal expansion or the thermal contraction becomes excessively large. Therefore, it is desired that the structure that is able to suppress an amount of change of the phase difference, which is caused due to the thermal expansion or the thermal contraction, sufficiently small should be implemented. In particular, a coefficient of linear expansion of a synthetic resin as the material of the sensor holder 7 is larger than that of a metal as material of other constituent members such as the outer ring 1, the hub 2, the rolling elements 3, 3, the cover 5, etc. As a result, it is desired that a structure capable of suppressing an amount of change of the phase difference, which is produced due to the thermal expansion or the thermal contraction of the sensor holder 7, should be implemented.
In other words, in the case of respective conventional structures, an end surface of the sensor holder 7 in the axial direction is brought into contact with an inner surface of the bottom plate portion 13 of the cover 5 in a state that the sensor holder 7 is held/fixed in an interior of the cover 5 by the mold forming or the adhesion. Therefore, the cylinder portion 14, which is holding the sensors 6a, 6b, out of the sensor holder 7 is thermally expanded or contracted in the axial direction from the side surface of the bottom plate portion 13 as an origin. Accordingly, for example, as shown in (A)→(B) of FIG. 22 or as shown by a thermal expansion in FIG. 24, both sensors 6a, 6b are displaced according to the thermal expansion or the thermal contraction mutually in the same direction along the axial direction. In contrast, the encoder 4 that is attached to the inner end portion of the hub 1 is hardly displaced in the axial direction (merely slightly displaced in contrast to both sensors 6a, 6b). As a result, it is possible to say that, in the case of respective conventional structures, the relative displacement between the sensed surface of the encoder 4 and the sensing portions of both sensors 6a, 6b in the axial direction is mainly caused based on the thermal expansion or the thermal contraction of the sensor holder 7.
As shown in above FIGS. 14 to 17, the above explanation is applied to the structure that senses either the relative displacement between the outer ring 1 and the hub 2 in the axial direction or the axial load acting between the outer ring 1 and the hub 2, based on the phase difference between a pair of sensors 6a, 6b being arranged in a state that the sensors 6a, 6b are separated in the axial direction. The phenomenon that the thermal expansion or the thermal contraction of the sensor holder 7 yields the measuring error may also occur in different degrees in respective cases of the above third example of the conventional structures shown in FIGS. 18 and 19, the above fourth example of the conventional structures shown in FIGS. 20 and 21, and the structure of the prior invention. For example, in the case of the third example of the conventional structure, when an amount of displacement caused based on the thermal expansion or the thermal contraction in the axial direction is varied between the sensors 6a, 6b, the measuring error is caused. Also, in the case of the fourth example of the conventional structure, the displacement of the sensor 6 caused based upon the thermal expansion or the thermal contraction in the axial direction yields the measuring error as it is. Therefore, as described above, it is desired that the structure capable of suppressing satisfactorily an amount of change of the phase difference caused by the thermal expansion or the thermal contraction of the sensor holder 7 should be implemented.
Patent Literature 1: JP-A-2006-113017
Patent Literature 2: JP-A-2006-317420
Patent Literature 3: JP-A-2007-93580